How much would a 500ml glass of lemon, lime and bitters typically cost?!

I don't think you can buy half a liter of citrus and bitters. Nor do I know why you'd want to.

Essentially you'd mostly be paying for the bitters, as that's the expensive bit. You can get lemon or lime juice for really cheap. Bitters would cost about 20 bucks for 500ml, give or take, so you'd be paying something around that at most.